1. Field of the Invention
This invention relates to wet strength resin compositions and methods of using them to produce cellulosic pulp fiber webs having increased wet strength.
2. Description of the Related Art
Polyamine-epichlorohydrin resins have been used as wet strength resins for paper since the early 1950's. These resins are cationic by virtue of the fact that they contain quaternary ammonium functionalities and are, therefore, substantive to negatively charged cellulose pulp fibers. These resins are particularly useful because they are formaldehyde-free and develop wet strength at neutral or alkaline pH values. One of the drawbacks associated with the use of a aminopolyamide-epichlorohydrin wet strength resins is the emission of harmful chlorinated compounds into the water systems of pulp and paper mills. These chlorinated compounds, which are the by-products of the manufacture of the aminopolyamide-epichlorohydrin resins, have been identified as epichlorohydrin, 1,3-dichloro-2-propanol, and 3-chloro-1,2-propanediol. A large percentage of these chlorinated organics, the total weight of any one or a combination of all of which is defined as the TOC1, are usually discharged into the air and water effluent from pulp and paper mills. Since permissible amounts of halogenated organics in waste waters is ever decreasing, considerable effort has been expended to reduce the amount of these materials in aminopolyamide-epichlorohydrin wet strength resins. Copending patent application Ser. No. 07/573,600, filed on Aug. 24, 1990 and now U.S. Pat. No. 5,350,796, provides a wet strength resin composition comprising from about 1% to about 60% by weight of a aminopolyamide-epichlorohydrin acid salt resin, up to about 0.3% by weight total organic chlorine or TOC1 based on the weight of said resin, and the remainder water. The aminopolyamide-epichlorohydrin acid salt resin in the wet strength resin has an E/N ratio of from about 0.6 to about 1.2. Prior to the present invention, it had been observed that compositions containing aminopolyamide-epichlorohydrin resins having TOC1 values in the 0.5 to 1.2% range did not increase wet tensile to the same degree as existing commercially available products which had E/N ratios .gtoreq.1.5. However, resins having E/N ratios .gtoreq. 1.5 also had TOC1 values which are too high for the lower contemporary TOC1 standards.
It is well known in the art to use retention aids or floculating agents to precipitate wet strength resins which by themselves are not substantive to pulp onto the surface of cellulosic pulp fibers when the wet strength resins are added at some point in the wet end of a paper machine during the paper making process. Some examples of wet strength resins that are not self retaining are neutral urea-formaldehyde resins, aldehyde-modified resins and dialdehyde starch dispersions. In each case, the cationic polymer serves to attract the wet strength resins by opposing electrostatic forces or by reducing the anoinic repulsive forces of cellulose fibers. Paper maker's alum is the simplest and perhaps the oldest material that has been used as a retention aid or flocculating agent. Most retention aids are positively charged materials which facilitate absorption onto the negatively charged surface of the cellulosic pulp fibers. Polymers having cationic charges are commonly used as retention aids. Examples include cationic urea-formaldehyde resins, cationic melamine-formaldehyde resins, cationic polyamine resins, cationic polyethyleneimine resins, cationic starch, polydiallyldimethylammonium chloride (polyDADMAC).
DE 3506832 teaches that paper having high dry strength and low wet strength is prepared by successive addition of water soluble cationic polymers and anionic polymers. The cationic polymers include the reaction product of an adipic acid-diethylenetriamine copolymer and epichlorohydrin, polyethyleneimine, and polydiallyldimethylammonium chloride (polyDADMAC). Anionic polymers include acrylamide-acrylic acid-acrylonitrile copolymers and acrylic acid-acrylonitrile copolymers. Nordic Pulp Paper Research J., 2, 49-55 (1987) teaches the rapid flocculation of Kraft fibers with dual-component retention aid systems comprised of, for example, polydiallyldimethylammonium chloride and polyacrylamide. U.S. Pat. No. 4,824,523 teaches a method for manufacturing paper comprising the step of adding a dry strength retention agent system to paper stock prior to forming paper wherein the system is comprised of cationic starch, an anionic polymer, non-starch cationic synthetic polymer, a cationic amido-amine-epichlorohydrin polymer, and a reaction product formed between epichlorohydrin or polyepichlorohydrin and ammonia. U.S. Pat. No. 4,754,021 teaches a method of enhancing the dewatering of paper during the papermaking process which includes adding a low molecular weight cationic organic polymer selected from polydiallyldimethylammonium chloride, and epichlorohydrindimethylamine copolymer. Canadian patent number 1,110,019 teaches a process for manufacturing paper having improved dry strength which comprises mixing an essentially alum free pulp slurry with a water soluble cationic polymer and subsequently adding a water soluble anionic polymer to the essentially alum free slurry and then adding alum. The water soluble cationic polymer can be the reaction product of epichlorohydrin and a polyamide-polyamine.
It is not known in the art to add cationic polymers or cationic polymers that do not yield wet strength properties into the wet end of a paper machine along with a self-retaining cationic wet strength resin such as a aminopolyamide-epichlorohydrin acid resin salt. The skilled artisan would seek to avoid the simultaneous use of two polymers having cationic charges because they would compete for adsorption onto the surface of the negatively charged cellulose fibers. Such a competitive adsorption situation would be expected to result in a lower than anticipated wet tensile increase. It is also not known in the art to resolve the low TOC1-low wet tensile building performance trade-off by employing a water soluble cationic polymer with a aminopolyamide-epichlorohydrin acid salt resin. The cationic polymer and aminopolyamide-epichlorohydrin acid salt resin can be combined into a composition and used to treat a pulp fiber slurry or the cationic polymer and aminopolyamide-epichlorohydrin wet strength resin can be added sequentially to the pulp fiber slurry.